Conventionally, scan correlation processing is used to eliminate sea level reflections (so-called “sea clutter”) in marine radar devices. In scan correlation processing, current sensed image data and written sensed image data of the same position that has been written one antenna rotation (one scan) before is used to determine the current written sensed image data. As a radar device performing such a scan correlation process, for example JP H11-352212A discloses a radar device performing scan correlation processing that is provided with an image memory storing sensed image data for one scan, and a write data generator that subjects the written sensed image data stored in the image memory and the current acquired sensed image data to predetermined data processing to generate new sensed image data.
FIG. 8 is a block diagram outlining the configuration of a conventional radar device. FIG. 9 is a block diagram illustrating the function of the write data generator of the radar device shown in FIG. 8.
As shown in FIG. 8, this conventional radar device includes a radar antenna 101 that, while rotating through the horizontal plane at a predetermined rotation period, sends out a pulse-shaped electromagnetic wave (send pulse signal) during a send period, and receives, in a polar coordinate system, electromagnetic waves (sense signals) reflected by an object during a receive period; a receiver portion 102 detecting a sensed signal; an A/D 103 sampling the sensed signal at a predetermined period and converting it into digital sensed data; and a sweep memory 104 into which the sensed data of one sweep are written in real time, and that outputs the data of this one sweep to a W data generator (write data generator) 107 before the sensed data obtained by the next send process are written again.
Moreover, this conventional radar device further includes a write address generator 105 and a FIRST/LAST detector 106. The write address generator 105 produces addresses specifying pixels in the image memory 108, arranged in a rectangular coordinate system, from the antenna angle θ (taking a predetermined direction as a reference) and the read-out position r of the sweep memory 104, taking the sweep rotation center as the start address and going around from the center. The FIRST/LAST detector 106 detects the timing at which, in one sweep rotation, a sweep has first accessed or last accessed each pixel in the rectangular coordinate system specified by the write address generator 105 in the image memory 108, and gives out a FIRST signal or a LAST signal.
The conventional radar device further comprises a W data (write data) generator 107, an image memory 108 and a display 109. The W data (write data) generator 107 performs a scan correlation between the sensed image data X(n) from the sweep memory 104 and the previous written sensed image data Y(n−1) at the timing of the FIRST signal or the LAST signal, and creates current written sensed image data Y(n). The written sensed image data are then written into the image memory 108 at the pixel addresses specified by the write address generator 105, and the written image data is displayed on the display 109.
The W data generator 107 of the conventional radar device has the function illustrated in FIG. 9, and determines the image data Y(n) to be currently stored in the image memory 108, in accordance with preset rules using the image data Y(n−1) stored pixel by pixel in the image memory 108 one antenna rotation before and the current image data X(n) that is input from the sweep memory. As an example of this rule, the image data Y(n) can be determined with the following rule:Y(n)=α·X(n)+β·Y(n−1)  (1)
Here, α and β are coefficients that are set in advance, in accordance with the purpose of the scan correlation process and the application.
In conventional radar devices performing a scan correlation process, the sensed image data that is currently to be written into the image memory (written sensed image data) is determined by the currently obtained sensed image data, the previously written sensed image data that is stored in the image memory, which is obtained from the sensed image data up to the previous scan, and coefficients for matching to a previously determined application. Therefore, obtaining a valid scan correlation process, that is, obtaining the most suitable effect for the scan correlation process is limited to the case that the previously set rules match the behavior of the sensed image data. For this reason, several types of scan correlation processing modes can be set, and the operator can switch to the most suitable processing mode for what is to be observed.
However, the conventional scan correlation process performs the same process on the entire processing range, so that if there are objects within that range that have different kinds of behavior, then it is not possible to concurrently perform an optimal scan correlation process for all those objects. That is to say, even if the scan correlation process is valid for a given portion within that range, it may conversely give poor results for other portions.
For example, if a buoy is recognized within sea clutter and another ship moving at a high speed is present as well, then the sea clutter can be suppressed, but that will also suppress quickly moving ships, so that the ship will become difficult to recognize. If, on the other hand, a scan correlation processing mode is selected with which other ships moving at high speed can be easily recognized, then the effect of removing sea clutter is diminished accordingly, so that the buoy becomes difficult to recognize.
Furthermore, if the state of the received echo or the state of the surroundings change as time passes or as the observation position (position of the own ship) moves, the operator needed to switch the optimal scan correlation processing mode in accordance with those changes.
Consequently, it is an object of the present invention to provide a radar and similar apparatus, that can reliably display sensed image data of objects, regardless of the state of the object within a sensed range and of the surrounding environment.